Approximately 430 ships are in active service or reserve for the U.S. Navy. The largest vessels, such as the Nimitz-class aircraft carrier, are over 1000 feet long. The fleet includes aircraft carriers, amphibious assault ships, cruisers, destroyers and submarines. Also included in the fleet are littoral combat ships and frigates. All U.S. Navy ships start with USS, for United States Ship, and as of October 2015, nuclear reactors power most aircraft carriers and submarines.